What is an Auxiliary Power Unit (APU)?

APS 3200 APU Design

Auxiliary power units (APU) are a device on a vehicle that provides energy for functions other than propulsion. They are usually found on large aircraft, as well as some large ground vehicles.

The main purpose of the aircraft Auxiliary Power Unit is to provide power for main engine start. Turbine engine must be accelerated to high rotational speeds in order to provide enough air compression for standalone operation. Smaller jet engines are usually started by an electric motor, while the larger machines are usually initiated by an air turbine motor. Before the machine should be turned, APU starts, generally by a battery or a hydraulic accumulator. Once the APU is running, it provides energy (electric, pneumatic, hydraulic or, depending on the design) to start the main engine aircraft.

Auxiliary Power Unit is also used to run accessories while the engine is off. This allows the cabin to be comfortable while the passengers boarding the aircraft before the engines started. Electricity used to run the system for preflight checks. Some APUs is also connected to a hydraulic pump, which allows the crew to operate the hydraulic equipment (such as flight control or flaps) before starting the engine. This function can also be used, on a plane a few, as a backup in flight in case of engine failure or hydraulic. In use, the APU provides auxiliary power for vehicles and convenience features added functionality and eliminates the high fuel consumption, wear, and emissions associated with long idle operation when on-board electricity generated by running the engine main drive.

Aircraft with APUs can also receive electrical power and pneumatic equipment from the ground when an APU has failed or not to be used.

APUs fitted to the extended-range twin engine operations (ETOPS) aircraft is a critical safety device, as they supply backup electricity and compressed air in the engine died or failed main engine generator. While some APUs may not start able in flight, ETOPS-compliant APUs must be start able flight at an altitude up to the aircraft service ceiling. Recent applications have specified starting up to 43,000 ft (13,000 m) of the complete immersion cold conditions such as Hamilton Sundstrand APS5000 for Boeing 787 Dreamliner. If the APU or generator power is not available, the aircraft cannot be released for ETOPS flight and is forced to take a longer route non-ETOPS.